Sensing system for detection and control of deposition on pendant tubes in recovery and power boilers

ABSTRACT

A system for detection and control of deposition on pendant tubes in recovery and power boilers includes one or more deposit monitoring sensors operating in infrared regions of about 4 or 8.7 microns and directly producing images of the interior of the boiler, or producing feeding signals to a data processing system for information to enable a distributed control system by which the boilers are operated to operate said boilers more efficiently. The data processing system includes an image pre-processing circuit in which a 2-D image formed by the video data input is captured, and includes a low pass filter for performing noise filtering of said video input. It also includes an image compensation system for array compensation to correct for pixel variation and dead cells, etc., and for correcting geometric distortion. An image segmentation module receives a cleaned image from the image pre-processing circuit for separating the image of the recovery boiler interior into background, pendant tubes, and deposition. It also accomplishes thresholding/clustering on gray scale/texture and makes morphological transforms to smooth regions, and identifies regions by connected components. An image-understanding unit receives a segmented image sent from the image segmentation module and matches derived regions to a 3-D model of said boiler. It derives a 3-D structure the deposition on pendant tubes in the boiler and provides the information about deposits to the plant distributed control system for more efficient operation of the plant pendant tube cleaning and operating systems.

This is related to U.S. Provisional Application No. 60/170,839 filed onDec. 14, 1999 and to PCT/US00/33879 filed on Dec. 14, 2000 and furtherto U.S. Ser. No. 10/168,277 filed on Jun. 14, 2002, all entitled“Sensing System for Detection and Control of Deposition on Pendant Tubesin Recovery and Power Boilers”.

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Contract No.DE-FC36-99G010375 by the Department of Energy.

This invention pertains to a sensing system for detection and control ofdeposition on pendant tubes in Kraft recovery boilers, and moreparticularly to a mid-band infra-red imaging system that is tuned to aparticular spectrum of infra-red radiation to see into the otherwisevisually impenetrable interior of a recovery boiler in operation toprovide information about the condition of pendant steam tubes in theboiler.

BACKGROUND OF THE INVENTION

In 1995, about 82% of the wood pulp consumed at US paper and paperboardplants was produced using the Kraft process. Although the proportion ofpulp from this source is likely to decline as new processes come online, it is expected that well over 50% of wood pulp production willstill be produced in 2020 using the Kraft process.

In the Kraft pulp production process, a fibrous material, most commonlywood chips, are broken down into pulp in a digester under pressure in asteam-heated aqueous solution of sodium hydroxide and sodium sulfide,called white liquor. After cooking in the digester, the pulp isseparated from the residual liquid called black liquor. Black liquor isan aqueous solution containing wood lignins, organic material, andinorganic compounds oxidized in the digester during the cooking process.It is concentrated and then burned in a recovery boiler to generatesteam, which is used in the pulp mill for pulp cooking and drying, andother energy requirements. The material remaining after combustion ofthe black liquor, called smelt, is collected in a molten bed at thebottom of the boiler and discharged to a dissolving tank to be recycledinto new white liquor.

Kraft chemical and energy recovery boilers, in which the black liquor isburned, are large and expensive, with capacities installed in the last30 years for pulp mills typically exceeding 1000 tons of pulp per day.It is difficult economically to add small incremental units of boilercapacity, so the capacity of the chemical recovery boiler is often thefactor limiting the capacity of the entire pulp mill.

The effective burning capacity of recover boilers is frequentlydetermined by the processes governing the deposition of fume,intermediate sized particles, and carryover of partially burntliquor/smelt drops on heat transfer surfaces of the steam and watertubes in the boiler, and the attendant plugging of gas passages betweenand around those pendant steam and water tubes. Much effort has beenmade and continues to be made to improving the understanding of themechanism of particulate and vapor deposition on the tubes. However,there are still no reliable on-line methods for systematically detectingthe presence and build-up rates of these deposits.

Various efforts to control the rate and quantity of deposits on thependant tubes in the boiler have been undertaken in the past. Theseinclude adjustments to conditions of combustion, such as the nozzlesthat spray the black liquor into the combustion chamber, and the way airis introduced into the combustion chamber. They also include systems,such as soot blowers, for removing deposits on the tubes before theyseriously impact the operation of the boiler. These control efforts aremost effective when they are immediately correlated to the results theyproduce, but heretofore there has been no reliable method of determiningdirectly the amount of deposits on the pendant tubes. Such controlefforts have therefore necessarily been based on indirect measurementsand considerations, and have usually yielded unsatisfactory results.

The severe environment of boilers, namely the high temperature,turbulent gas flow, particle laden atmosphere, and intensity ofradiation have made it difficult to develop a sensing system fordetection and control of deposition on pendant tubes in Kraft recoveryboilers that would be economically viable as a commercial product.Attempts to use near-IR cameras for direct monitoring of pendant tubedeposits have failed to reliably produce good images over the span oflarge boilers, and devices operating at longer wavelengths have beenimpractical for boiler-side use because of prohibitive expense and theneed for reliable cryogenic cooling.

U.S. Pat. No. 4,539,588 entitled “Imaging of Hot Infrared EmittingSurfaces Obscured by Particulate Fume and Hot Gasses” issued on Sep. 3,1985 to Peter C. Ariessohn and R. K. James discloses an improvement inthe technology of the time, but operated in a wavelength region of1.5-1.8 micron, which has a relatively high susceptibility to lightscattering by particles in the boiler gas stream.

Thus, there has long been a serious need for a deposition detectionsystem for recovery boiler pendant tubes to solve the unfulfilledrequirement to monitor the degree and distribution of fume, intermediatesized particles, and carryover particle depositions on recovery boilertubes.

SUMMARY OF THE INVENTION

Accordingly, this invention provides a method of directly monitoring thedepositions on recovery boiler pendant tubes.

The invention includes a focal plane array camera capable of creatingimages in a particular range of infrared radiation that has lowabsorption by molecules in the gas stream in a chemical recovery boiler,and is not scattered significantly by particles normally present in theboiler gas stream. Another aspect of the invention is a system of one ormore deposit monitoring sensors feeding signals to a data processingsystem under control of a distributed control system. Preferably, thedeposit monitoring sensors include focal plane array cameras operatingin the mid-infra-red band, in the region of about 4-12 micronswavelength. Clear images can be obtained at a low cost of the boilerinterior and particularly of the pendant water and steam tubes in theboiler to enable for the first time a visual real time inspection of thecondition of the tubes and depositions thereon so that control schemescan be implemented.

DESCRIPTION OF THE DRAWINGS

The invention and its many attendant features and advantages will becomeclear upon reading the following detailed description of the preferredembodiment, in conjunction with the following drawings, wherein:

FIG. 1 is a schematic diagram of the invention installed in a Kraftrecovery boiler;

FIG. 2 is a schematic elevation of a monitoring sensor shown in FIG. 1;

FIG. 3 is an elevation of a hand-held sensor in accordance with thisinvention;

FIG. 4 is an elevation of the hand-held sensor of FIG. 3 showing the airflow system for the lens tube;

FIG. 5 is a sectional end elevation of the a monitoring sensors shown inFIG. 2;

FIG. 6 is a sectional elevation of the a monitoring sensor shown in FIG.3 along lines 6-6 in FIG. 4;

FIG. 7 is an enlarged sectional elevation of the distal end of the amonitoring sensor shown in FIG. 6;

FIG. 8 is a schematic diagram of the optical elements in the sensorshown in FIG. 3;

FIG. 9 is a graph showing the light transmission over a range ofwavelengths in a recovery boiler; and

FIG. 10 is a schematic flow diagram of the process of receiving datafrom the monitoring sensors in FIGS. 1 and 3 to data input to thedistributed control system in FIG. 1 for control of deposition controlsystems in the boiler.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings, and more particularly to FIG. 1 thereof, adeposition detection system in accordance with this invention is showninstalled in a Kraft recovery boiler 30. The deposition detection systemincludes one or several monitoring sensors 35, described in detailbelow, which acquire data in the midband infra-red spectrum within aparticular field of view from the interior of the recovery boiler. Themonitoring sensors 35 could be fixed in position to permanently monitorparticular areas within the boiler, or could be integrated hand-heldunits 36 shown in FIG. 3.

The sensor 35, shown in more detail in FIGS. 2 and 5-7, converts theacquired infrared data to electrical signals, which are conducted viaelectrical lines 37 to a sensor data processing system 40, shown in moredetail in FIG. 10 and described in detail below. The hand-held sensor 36shown in FIG. 3 converts the acquired infrared data directly to an imagethat is viewed on a display inside a hood 42 at the viewer end of acamera body 44.

A distributed control system 45, which is the computer system used bypaper mill or power plant operators for controlling the operation of theplant, is connected to the sensor data processing system 40 by a link 47for advanced control of the boiler operations in accordance with theinformation supplied by the sensor data processing system 40, withoperator judgement and analysis where necessary, to minimize depositionof the pendant steam tubes 49 and otherwise maximize plant efficiency.

Turning now to FIG. 2, one embodiment of the deposit monitoring sensor35 is shown having a focal plane array camera 50 and a lens tube 55connected to the camera 50 by way of a spectral band-pass filter 60 thatlimits the light admitted into the camera 50 to a particular band in themid-IR spectrum.

The imaging optics consist of the double-layered steel lens tube 55,shown in FIGS. 5-7, containing a train of ZnSe or, amorphous siliconlenses 65. The front lens 70 serves as the objective, and has a field ofview of at least 20°. Three other lenses 75, 76 and 77 serve as transferoptics, transporting the image formed by the objective onto the focalplane array 78 of the camera 50. The electrical signals from the imagingarray 78 are processed in the electronic circuitry 79 and transmitted toa remote processing system, in the case of the monitoring system shownin FIG. 2, or displayed on a display such as an LCD display screen 80 inthe case of the hand-held unit shown in FIG. 3. The total length of thelens tube is about 36 inches, permitting the focal plane to be locatedremotely from the boiler port. The lens tube 55 is cooled and purged bya constant stream of air supplied at about 30 psi through a gas coupling81 into the space between the inner and outer tubes of thedouble-layered lens tube 55, through which the air flows and exits outthrough an axial opening 85 at the distal end of the lens tube 65.

Several camera models could be used: a ferroelectric array camera, aPtSi camera, and a Si microbolometer array camera. Also, an InSb arraycamera operating in the 3.9 micron wavelength region, has producedadequate images but was determined to be impractical because of its costand the limited lifetime of the necessary low temperature coolingsystems required for operation of the camera. The ferroelectric arraycamera is attractive because it does not require cryogenic cooling, doesnot require frequent calibration and is relatively inexpensive. However,it does use a semi-transparent “chopper” wheel to limit the intensity ofthe light to the array. The chopper wheel introduces its own set ofproblems such as the superimposition of artifacts such as curved linesacross the image. These problems can be addressed by changing requiredsolutions to achieve satisfactory images. The PtSi array camera requirescryogenic cooling and is quite expensive, making it a less preferredversion of the usable cameras. The microbolometer array camera does notrequire cryogenic cooling and does not use a chopper. It also hassignificantly greater dynamic range than the ferroelectric array camera.However, it may require frequent (once-a-day) re-calibration to produceacceptable images, and is significantly more expensive than theferroelectric array camera.

The preferred camera is a ferroelectric array camera modified to viewinfrared radiation in a wavelength band of about 3.5-4.0 microns,preferably about 3.9 microns; or infrared radiation in a wavelength bandof about 8.5-9.0 microns, preferably about 8.7 microns. This cameraproduces clear images in the system outlined above and is inexpensiveenough to be affordable for pulp mills to purchase and use. Weanticipate that other imaging arrays usable in our camera will bedeveloped that will be usable in the system shown in FIG. 1

The sensor shown in FIG. 3 includes the camera body 44 connected to thelens tube 65 by way of an intermediate structure 85. The intermediatestructure 85 includes an adjustable iris 90 and the lens 60, which isaxially movable to give the lens train the ability of focus in alow-light, wide aperture condition. The hand-held unit 37 has a powerswitch 94 and an electrical connector 96, which provides the ability toconnect electronically into the distributed control system 45. The lenstube 55 is connected to a source 98 of air pressure through a pressureregulator 97 and a flexible air hose 99.

The graph on FIG. 9 illustrates the benefits of operating in the regionsof about 4 and 8.7 microns. As illustrated, there are several “windows”available to viewing the interior of a chemical recovery boiler byvirtue of the light absorption characteristics of the gas and vapors inthe gas stream of a chemical recovery boiler for a pulp mill. Thevisibility of the boiler interior at these wavelengths is alsoinfluenced by the scattering effect of the particles in the boiler gas.The effectiveness of this particle scattering is greatly decreased atlonger wavelengths, and for wavelengths in excess of 3 microns does notsignificantly degrade images of recovery boiler interiors in the upperfurnace and convection-pass sections. By operating an a region of lowabsorption and low scatter of the gas molecules and particles,respectively, in the boiler gas stream, the resolution of the imagesthat are possible by infrared imaging in the chemical recovery boiler ismaximized.

Turning now to FIG. 10, an image processing system 40 and a link to oneversion of the distributed control system is shown having a video input90 from the camera 50 to an image pre-processing circuit 95 in which the2-D image is captured and noise filtering is performed in a low passfilter. Array compensation is accomplished to correct for pixelvariation and dead cells, etc., and geometric distortion is corrected byimage system compensation. A cleaned image 100 is sent from the imagepre-processing circuit to an image segmentation module 105 where theimage of the recovery boiler interior is separated into background,pendant tubes, and deposition. Thresholding/clustering on grayscale/texture is accomplished and morphological transforms to smoothregions are made. Regions are identified by connected components. Thesegmented image 110 is sent from the image segmentation module 105 to animage-understanding unit 115 where derived regions are matched to a 3-Dmodel of the recovery boiler and a 3-D structure 120 of the depositionis inferred. Those deposition estimates can be provided to thedistributed control system to update the computer model and state 125 ofthe recovery boiler which is fed back in a closed loop to continuallyupdate the image understanding unit 115. The deposition estimates 120are fed to the “soot-blower” control 130 for optimized control of thesteam cleaning system for the pendant tubes 49 in the boiler.

A control scheme is envisioned that utilizes the information from thedeposition detection system to control or minimize further deposition,or optimize deposit removal processes. From the processed images, thesystem identifies the location of deposits and activates the steamcleaners, or “soot-blowers”, that are most appropriate to clean theaffected location and prevent pluggage. Currently, the “soot-blowers”are operated “blind” on a timed cycle. Operating only the soot-blowersonly where and when there are deposits needing removal will minimize thesteam usage as well as tube wear caused by unnecessary over-cleaning.Moreover, it is now possible for the first time to accurately relate thedeposition rate to the liquor burning parameters, so the boileroperation can be optimized to minimize deposits on the pendant tubes.

Obviously, numerous modifications and variations of the preferredembodiment described above are possible and will become apparent tothose skilled in the art in light of this specification. For example,many functions and advantages are described for the preferredembodiment, but in some uses of the invention, not all of thesefunctions and advantages would be needed. Therefore, we contemplate theuse of the invention using fewer than the complete set of notedfunctions and advantages. Moreover, several species and embodiments ofthe invention are disclosed herein, but not all are specificallyclaimed, although all are covered by generic claims. Nevertheless, it isour intention that each and every one of these species and embodiments,and the equivalents thereof, be encompassed and protected within thescope of the following claims, and no dedication to the public isintended by virtue of the lack of claims specific to any individualspecies. Accordingly, we expressly intend that all these embodiments,species, modifications and variations, and the equivalents thereof, areto be considered within the spirit and scope of the invention as definedin the following claims, wherein we claim:

1. A sensing system for detection and control of deposition on pendanttubes in recovery and power boilers, comprising: at least one depositmonitoring sensor feeding signals to a data processing system forgenerating information about deposition on said pendant tubes to enablea distributed control system by which said boilers are operated tooperate said boilers more efficiently.
 2. A deposit monitoring sensorfor monitoring deposits on pendant tubes in a Kraft recovery boiler or apower boiler, comprising: an elongated lens tube containing high IRtransmissivity optic elements and a spectral band-pass filter forpassing radiation in a preferred wavelength band, coupled to a camerasensitive to said wavelength band, said lens tube having a coolingchannel to prevent excessive heating of said optic elements from theboiler.
 3. A deposit monitoring sensor as in claim 2, wherein saidcooling channel comprises: an inner wall of said lens tube in whichlenses are mounted, and an outer wall surrounding said inner wall; aninlet fluid coupling at one end of said outer tube for delivering a flowof cooling fluid between said inner wall and said outer wall of saidtube; an outlet in said outer tube for conveying said cooling fluid outof said outer tube through said outlet for cooling to prevent excessiveheating from the boiler.
 4. A deposit monitoring sensor as in claim 2,wherein said cooling channel comprises: an inner wall of said lens tubehas in which lenses are mounted, and an outer wall surrounding saidinner wall, said outer wall having an open distal end; an air couplingfor connection to a source of air pressure for delivering a flow ofcooling air between said inner wall and said outer wall of said tube andfor discharging said cooling air flow out through said opening at saiddistal end of said outer wall; whereby a flow of cooling air isestablished along said lens tube for cooling to prevent excessiveheating from the boiler.
 5. A deposit monitoring sensor according toclaim 2 wherein said optic elements comprise zinc selenide or amorphoussilicon materials.
 6. A deposit monitoring sensor according to claim 2wherein said preferred wavelength band is in the region of 4 microns. 7.A deposit monitoring sensor according to claim 2 wherein said preferredwavelength band is in the region of 8.7 microns.
 8. A deposit monitoringsensor according to claim 2 wherein said camera includes a focal planearray.
 9. A deposit monitoring sensor according to claim 2 wherein: saidcooling channel in said lens tube includes an inner wall and an outerwall surrounding and spaced from said inner wall and definingtherebetween an annular space for flow of cooling fluid for conveyingheat from said outer wall and protecting said inner wall and said lenstube from excessive heating from hot gasses in said boiler.
 10. Adeposit monitoring sensor according to claim 9 wherein: said coolingchannel in said lens tube includes a coupling for connecting a source ofair pressure to said lens tube for establishing a flow of cooling airaround said inner tube.
 11. A deposit monitoring sensor according toclaim 9 wherein: said cooling fluid is water.
 12. A deposit monitoringsensor according to claim 10 wherein said cooling channel in said lenstube includes an opening at the distal end of said tube through whichsaid cooling fluid exits said lens tube. 13-18. (canceled)
 19. A methodof determining deposition loading on pendant tubes in a chemical recoverboiler, comprising: producing a real-time image of said pendant tubes inan imaging system; measuring periods of oscillation of said tubes whennewly installed after perturbation thereof by factors such assoot-blowers which cause said pendant tubes to swing; remeasuringperiods of oscillation of said tubes after periods of operation in saidboiler, and after some amount of deposition on said tubes, afterperturbation thereof by factors such as soot-blowers which cause saidpendant tubes to swing; comparing periods of oscillation of said tubeswhen clean and after deposition; calculating changes in mass that wouldaccount for changes in periods of said oscillation from said deposition.